1. Field of the Invention
The present invention relates generally to portable visual examination apparatus and, in particular, to a hand-held instrument for bleaching the macula of the human retina and measuring its recovery time.
2. Description of the Prior Art
The retina of the human eye consists of several layers, including a layer of nerve cells and fibers and a deeper layer of rods and cones. Cones differ from rods in that the cones provide a human with color and detailed vision, whereas rods differentiate black/white and motion. This layer of rods and cones is the light-sensitive layer. Located in a small central area of the retina is the macula, which houses the higher concentration of cones. Further, within the macula, the highest concentration of cones resides in the fovea.
Macular photostress testing is a sensitive method for detecting patients with macular disturbances, such as cystoid macula edema, central serous choroidopathy and senile macular degeneration. These disturbances have been shown to cause a significantly prolonged recovery from photostress.
It is well known in the art that exposing the macula of the human retina during an ophthalmoscopic examination to a bright light requires a certain amount of time for the human eye to recover useable visual acuity. This return time is commonly referred to as "recovery" time and varies from individual to individual. In many individuals, an unusually long recovery time is encountered after this photostressing or bleaching. This lengthened recovery time may be an indication of subliminal or developing maculopathy or other retinal disease. A recovery rate longer than normal helps the doctor identify persons at risk for developing macular degeneration or other retinal disease. In addition, a photostress test is particularly helpful in following intra-ocular surgery when cystoid macular edema is suspected. An increase in or lengthy recovery time indicates a maculopathy and helps to rule out other complicating factors, such as irregular astigmatism, early capsular opacification or optic nerve disease.
In assessing recovery time, there are various methods for definitively marking the point of re-focus (or recovery). A Snellen Eye Chart or an Amsler grid are two examples of tools used in marking the point of re-focus. A Snellen Eye Chart is a well-known chart consisting of lines of gradually reduced-sized letters. Typical Amsler grids, with focusing improvements, are disclosed in U.S. Pat. No. 5,646,710 to Caskey and U.S. Pat. No. 5,838,422 to Caskey. Further, U.S. Pat. No. 5,139,030 to Seay, Jr. discloses an Amsler grid with an illuminated center to assist the patient in focusing on the center of the grid. A Snellen Eye Chart may be utilized in determining the recovery time between bleaching of the retina to the return of visual acuity. While the Amsler grid may be used to assess recovery time, it is typically used as a screening device to ascertain loss or distortion of central or macular vision.
A flash unit, a timer and a focusing device have been previously combined together to create a visual examination apparatus for testing for macular degeneration. For example, U.S. Pat. No. 4,545,658 to Weiss discloses the use of such a combination for administering the photostress recovery test. However, the disadvantage of the Weiss apparatus is its requirement to be housed in a large, non-portable cabinet. While this cabinet-enclosed photostress instrument assists in normalizing the photostress procedure and simplifying the focusing process, the overall unit is large, cumbersome and is burdened with a split imaging mirror. In addition, such an instrument requires standalone optotype slides and continual administrative interaction.